P2P code test coverage

**User Story**
As a developer or user I want to see more of the P2P networking code covered by tests in order to increase my confidence in the software.

**Impacts**
- [ ] API (the application programming interface)
- [ ] Build (the build process or something prior to compiled code)
- [ ] CLI (the command line wallet)
- [ ] Deployment (the deployment process after building such as Docker, Travis, etc.)
- [ ] DEX (the Decentralized EXchange, market engine, etc.)
- [x] P2P (the peer-to-peer network for transaction/block propagation)
- [ ] Performance (system or user efficiency, etc.)
- [ ] Protocol (the blockchain logic, consensus, validation, etc.)
- [ ] Security (the security of system or user data, etc.)
- [ ] UX (the User Experience)
- [x] Other (please add below)
Code quality

**Additional Context (optional)**
See also https://github.com/bitcoin/bitcoin/issues/14210

Ideas:
[ ] unit tests
[ ] smoke test (during Travis build)
[ ] network stress test
[ ] protocol simulator

## CORE TEAM TASK LIST
- [ ] Evaluate / Prioritize Feature Request
- [ ] Refine User Stories / Requirements
- [ ] Define Test Cases
- [ ] Design / Develop Solution
- [ ] Perform QA/Testing
- [ ] Update Documentation

Start by surveying the C++ P2P networking code and its existing tests; the issue does not identify files, entry points, or specific behaviors. Define test cases and coverage targets for the P2P code, then verify that the added unit, smoke, stress, or simulator tests run successfully and improve coverage.

Written by the indexing model from the issue text.